1. Field of the Invention
The present invention relates to an acoustic wave filter used in mobile communication equipment and the like.
2. Description of the Related Art
As a conventional acoustic wave filter, as shown in FIG. 19, for example, a configuration in which three acoustic wave resonators D, E, and F are connected in series has been known. With such a configuration, a voltage applied to each pair of comb-shaped electrode fingers is reduced and a mechanical stress intensively applied is distributed so as to improve power durability (see Patent Document 1).
However, such a conventional acoustic wave filter in which a plurality of stages of acoustic wave resonators are connected in series has a problem that loss due to spurious responses is large.
When acoustic wave resonators D, E, and F having an equal overlap width are cascade-connected, if a transverse mode spurious response occurs in acoustic wave resonators D, E, and F, respectively, the spurious responses appear in the same frequency with respect to acoustic wave resonators D, E, and F. Consequently, the transverse mode spurious responses of acoustic wave resonators D, E, and F are strengthened by each other, thus increasing loss in the pass band.
This state is shown in FIGS. 20 and 21. FIG. 20 is a graph showing an admittance characteristic of the conventional acoustic wave resonator shown in FIG. 19. FIG. 12 is a graph showing a pass characteristic of the acoustic wave resonator. As shown in these graphs, deep transverse mode spurious responses S are largely generated between resonance frequency A and anti-resonant frequency B. As a result, loss in the pass band becomes large in the portions in which transverse mode spurious responses S are generated.
Furthermore, as a conventional acoustic wave filter, as shown in FIG. 22, for example, a configuration in which three acoustic wave resonators G, H, and I are connected in parallel is known. With such a configuration, the bandwidth of the pass band can be widened (see Patent Document 2).
However, such a conventional acoustic wave filter in which a plurality of stages of acoustic wave resonators are connected in parallel also has a problem that loss due to spurious responses is large.
When acoustic wave resonators G, H, and I having an equal overlap width are connected in parallel, if a transverse mode spurious response occurs in acoustic wave resonators G, H, and I, respectively, the spurious responses appear in the same frequency with respect to acoustic wave resonators G, H, and I. Consequently, the transverse mode spurious responses of acoustic wave resonators G, H, and I are strengthened by each other, thus increasing loss in the pass band.
This state is shown in FIGS. 23 and 24. FIG. 23 is a graph showing an admittance characteristic of the conventional acoustic wave resonator shown in FIG. 22. FIG. 24 is a graph showing a pass characteristic of the acoustic wave resonator. As shown in these graphs, deep transverse mode spurious responses S are largely generated between resonance frequency A and anti-resonant frequency B. As a result, similar to the case of cascade connection, loss in the pass band becomes large in the portions in which transverse mode spurious response S are generated.    Patent Document 1: Japanese Patent Unexamined Publication No. H9-205343    Patent Document 2: Japanese Patent Unexamined Publication No. 2000-77972